1. Field of the Invention
The present invention relates to a- disc discriminator of an optical disc player system, and more particularly, to an apparatus for automatically discriminating between a digital video disc (DVD) and a compact disc (CD) installed in a digital video disc/compact disc (DVD/CD) compatible player system. The present application is based on Korean Application No. 11966/1996, which is incorporated herein by reference.
2. Description of the Related Art The growth of multi-media systems drives the progress in optical disc player systems. For example, a recently developed DVD [or SD (Super Density Disc)] player is capable of recording and reproducing a moving image and an audio signal of high quality. The DVD satisfies the MPEG-2 (Motion Picture Experts Group-2) specification and is compatible with the video CD based on the MPEG-1(Motion Picture Experts Group-1) specification. The thickness of the DVD substrate was chosen as 0.6 mm in consideration of the mechanical strength of the substrate against deformation of the disc and the compatibility with the CD. The thickness of the general CD is 1.2 mm. The compatibility between the DVD and the CD is motivated by the fact that much of the software that has been developed and is available on the market uses the format of the CD.
In order to reproduce information recorded in the DVD and the CD by means of a single optical disc player system, an optical pickup unit for accurately reading information recorded on either a disc having a substrate thickness of 0.6 mm or a disc having a substrate thickness of 1.2 mm is required. Additionally, a disc discriminator for identifying the kind of the disc loaded in the player is necessarily required. The DVD/CD compatible optical pickup units are classified into two systems, one class uses a liquid crystal shutter and the other class uses a hologram element.
FIGS. 1A and 1B are diagrams illustrating a disc reading operation using the DVD/CD compatible optical pickup unit employing the liquid crystal shutter. As shown in FIGS. 1A and 1B, the DVD/CD compatible optical pickup unit using the liquid crystal shutter is capable of changing the diameter of the laser beam projected on the objective lens. FIG. 1A shows the case wherein the DVD having the substrate of thickness of 0.6 mm is installed and the liquid crystal shutter is not driven, i.e., the liquid crystal shutter is off. FIG. 1B shows the case wherein the CD having the substrate of thickness of 1.2 mm is installed and the liquid crystal shutter is on.
Referring to FIGS. 1A and 1B, a reference numeral 12 indicates a laser beam generated from the semiconductor laser generator (hot shown) and a reference numeral 14 indicates an objective lens. A reference numeral 16 denotes a liquid crystal shutter which is used for changing the diameter of the laser beam and projecting it on the objective lens 14 by means of an xe2x80x9con/offxe2x80x9d operation.
The above principle depends on the fact that the focal distance of the objective lens 14 differs according to the diameter of the laser beam projected thereon. For example, as shown in FIG. 1A, when the liquid crystal shutter 16 is off, the diameter of the laser beam 12 is about the same as the diameter of the objective lens 14, and the focal distance of the objective lens 14 becomes short. If the liquid crystal shutter 16 is turned on, the diameter of the laser beam projected on the objective lens 14 becomes narrower. Thus, the focal distance of the objective lens 14 increases.
As can be understood from the above, the liquid crystal shutter 16 should be appropriately driven according to the kind of the disc to be reproduced. That is, it is necessary to identify the kind of disc installed in the optical disc player system. The discrimination between the discs inserted in the optical disc player is achieved by detecting the level of a focusing search signal picked up from the disc by means of the optical pickup unit. The focusing search signal appearing as a form shown in FIGS. 2A and 2B is obtained from a focusing error signal FE. Thus, conventionally the focusing error signal FE is also used for discriminating between the discs by comparing its level to a reference voltage as will be further explained below.
FIGS. 2A and 2B are wave forms of the focusing search signal while the DVD/CD is reproduced by switching off and on the liquid crystal shutter shown in FIG. 1. Referring to FIGS. 2A and 2B, a micro meter xcexcm in a horizontal axis denotes focus inconformity and a volt xe2x80x9cVxe2x80x9d in a vertical axis denotes the amplitude of the focusing search signal.
FIG. 2A illustrates a wave form of the focusing search signal while the DVD having the thickness of 0.6 mm is being reproduced by switching off the liquid crystal shutter 16 of the DVD/CD compatible optical pickup unit. FIG. 2B illustrates a wave form of the focusing search signal while the CD of 1.2 mm is being reproduced by switching on the liquid crystal shutter 16 of the DVD/CD compatible optical pickup unit. When the CD of 1.2 mm is reproduced by switching on the liquid crystal shutter 16 of the DVD/CD compatible optical pickup unit, the level of the focusing search signal becomes low and the degree of the focus inconformity becomes large, owing to spherical aberration. That is, as shown in FIG. 1A, if the DVD disc is replaced by a CD while the liquid crystal shutter 16 is maintained off, the laser beam passing through the objective lens is projected on the recording surface of the CD. However, since the CD is twice as thick as the DVD, spherical aberration reduces the level of the pickup signal.
FIG. 3 is a block diagram of a conventional disc discriminator which is used for explaining how the DVD/CD player discriminates between the discs using the focusing search signal shown in FIGS. 2A and 2B. There are provided a 4-split photo-detector 18, first and second adders 20 and 22, a differential amplifier 24, and a comparator 26. The 4-split photo-detector 18 consists of first to fourth quadrantal photo diodes a, b, c, and d for receiving the laser: beam picked up through the objective lens 14, the liquid crystal shutter 16 and a beam splitter (not shown), and generating an electrical signal. The first adder 20 adds the output of the first quadrantal photo diode a of the 4-split photo diode 18 to an output of the third quadrantal photo diode c thereof. The second adder 22 adds the output of the second quadrant photo diode xe2x80x9cbxe2x80x9d to an output of the fourth quadrantal photo diode d. The differential amplifier 24 differential-amplifies a difference between the outputs of the first and second adders 20 and 22 and then generates the focusing error signal FE. The comparator 26 compares the focusing error signal FE to a predetermined reference voltage Vref and then generates a disc discrimination signal DVD/CD-S of the DVD/CD. The reference voltage Vref in the comparator 26 is set up to discriminate the level of a focus signal of the focusing search signal when the DVD and the CD are reproduced as shown in FIGS. 2A and 2B. The reference voltage Vref is lower than the level of the focusing search signal when the DVD is reproduced, and it is higher than the level of the focus signal of the focusing search signal when the CD is reproduced. Here, all constructions, except the comparator 26, are being currently applied to the generally produced CD players. Thus, in the following description, there will be explained within the scope of the present invention only an operation to discriminate between the DVD and the CD.
If the information recorded in the disc is reproduced under a situation that the liquid crystal shutter 16 of the DVD/CD compatible optical pickup unit is off, the laser beam reflected from the disc is projected onto the upper surface of the 4-split photo-detector 18 shown in FIG. 3. At this time, the form of the laser beam differs according to the tracking state of the optical pickup unit. The first adder 20 adds the signals generated from the first and third quadrantal photo diodes a and c of the 4-split photo-detector 18 and then generates an added signal xe2x80x9ca+cxe2x80x9d. The second adder 22 adds the signals generated from the second and fourth quadrantal photo diodes b and d of the 4-split photo-detector 18 and then generates an added signal xe2x80x9cb+dxe2x80x9d. The differential amplifier 24 receives the added signals generated from the first and second adders 20 and 22 and amplifies a difference xe2x80x9c(a+c)xe2x88x92(b+d)xe2x80x9d between the two signals and then generates the focusing error signal FE which is used as the focusing search signal like FIG. 2A or FIG. 2B. The detected focusing search signal is supplied to a comparing voltage terminal of the comparator 26 to which the predetermined reference voltage Vref is applied. The comparator 26 compares the reference voltage Vref with a voltage of the detected focusing search signal. If the voltage of the focusing search signal is higher than the reference voltage Vref, the comparator 26 generates a DVD disc discrimination signal, for example, the logic xe2x80x9chighxe2x80x9d through an output terminal DVD/CD-S. On the other hand, if the voltage of the focusing search signal is lower than the reference voltage Vref, the comparator 26 generates a signal at the logic xe2x80x9clowxe2x80x9d, which is used for indicating that the CD is reproduced.
However, the compatible DVD/CD discriminator shown in FIG. 3 is inadequate when applied to a system using the hologram element. The problem will be explained hereinafter.
FIGS. 4A and 4B are diagrams illustrating how a disc reproducing operation is performed using the conventional DVD/CD compatible optical pickup unit using the hologram element. A reference numeral 28 denotes a hologram lens having a hologram lattice pattern. FIG. 4A shows an operating state when reproducing the DVD of 0.6 mm. FIG. 4B shows an operating state when reproducing the CD of 1.2 mm. As shown in FIGS. 4A and 4B, the DVD/CD compatible optical pickup unit using the hologram element diffracts the laser beam 12 generated from the semiconductor laser generator (not shown) by means of a lattice pattern 30 of the hologram lens 28 and projects it on the objective lens 14, thereby changing the focus distance of the objective lens 14. The lattice pattern 30 of the hologram lens 28 is formed in the shape of sawedged prominence and depression.
In FIG. 4A, the laser beam 27 which is not diffracted in the hologram lens 28 is focused on the DVD of 0.6 mm by means of the objective lens 14, so that the information recorded on the DVD is reproduced. Further, the laser beam 29 which is diffracted by the hologram lattice pattern 30 of the hologram lens 28 is projected on the objective lens 14, as shown in FIG. 4B. The focus distance of the objective lens 14 becomes longer as shown in FIG. 4B. At this time, if the intensity of the laser beam diffracted by the hologram lattice pattern 30 is lowered, the focusing search signal during the reproduction of the DVD and the CD is as shown in FIGS. 5A and 5B, so that the type of the disc installed can be identified by using the circuit of FIG. 3. However, the optical efficiency of the optical pickup unit using the hologram lens reaches to just about 20% of the general optical pickup unit. Thus, many problems may arise owing to the low intensity of the light reflected from the CD. In order to prevent the reduction in intensity, the lattice pattern of the hologram element 28 is adjusted to relatively increase the intensity of the primary diffracted beam from the hologram lens 28.
When the DVD and the CD are reproduced by increasing the intensity of the primary diffracted beam, each of the focusing search signals of the DVD and the CD becomes large as shown in FIGS. 6A and 6B. However, if the intensity of the primary diffracted beam is increased, the levels of the focusing search signals of the two discs are similar to each other. Consequently, it is difficult to discriminate between the DVD and the CD by the circuit of FIG. 3. Further, when the intensity of the primary optical beam diffracted from the hologram lens 28 is high, it is hard to set up the reference voltage Vref supplied to the comparator 26 so as to discriminate between the DVD and the CD, since the levels of the focusing search signals are similar to each other during the reproduction of the DVD and the CD.
It is therefore an object of the invention to provide a circuit for accurately identifying the type of a disc loaded in an optical player system having a DVD/CD compatible optical pickup unit using a hologram element.
It is another object of the invention to provide a circuit for automatically discriminating between a DVD and a CD by detecting a tracking error signal of a push-pull method.
It is still another object of the invention to provide a disc discriminator of an optical disc player system using a DVD/CD compatible optical pickup unit of a hologram element wherein the focusing search signals for 0.6 mm and 1.2 mm discs have similar level.
In order to achieve these objects of the present invention, a disc discriminator of an optical disc player system having an optical pickup unit of a hologram element for focusing an incident laser beam onto a disc and concentrating the beam reflected therefrom includes: a 4-split photo-detector for converting the beam concentrated by the optical pickup unit into an electric signal by means of a plurality of photo diodes divided in first through fourth quadrants; a push-pull signal generator for amplifying a difference between signals generated by two pairs of photo diodes which are symmetrical in the upper and lower sides from the center of a track direction among the 4-split photo-detector, and generating a push-pull signal; and a comparator for comparing the push-pull signal generated from the push-pull signal generator with a predetermined reference voltage, and generating a digital video disc/compact disc discrimination signal discriminating between a digital video disc having the thickness of 0.6 mm and a compact disc having the thickness of 1.2 mm.